class Solution {
public:
    int maxProfit(vector<int>& prices) {
        if (prices.size() < 2) return 0;
        int n = prices.size();
        vector<vector<int>> dp(n, vector<int>(5));
        dp[0][0] = 0; //不操作
        dp[0][1] = -prices[0]; //第一次持有
        dp[0][2] = 0; //第一次不持有
        dp[0][3] = -prices[0]; //第二次持有
        dp[0][4] = 0; //第二次不持有
        for (int i = 1; i < n; i++) {
            dp[i][1] = max(dp[i - 1][1], dp[i - 1][0] - prices[i]);
            dp[i][2] = max(dp[i - 1][2], dp[i - 1][1] + prices[i]);
            dp[i][3] = max(dp[i - 1][3], dp[i - 1][2] - prices[i]);
            dp[i][4] = max(dp[i - 1][4], dp[i - 1][3] + prices[i]);
        }
        return max(dp[n - 1][2], dp[n - 1][4]);
    }
};
